Thermoplastic polyamides, such as nylon 6 and nylon 6,6, are a class of materials which possess a good balance of properties comprising good elongation, high strength, high energy to break and stiffness which make them useful as structural materials. However, thermoplastic polyamides are quite sensitive to crack propagation. Consequently, a major deficiency of thermoplastic polyamides is their poor resistance to impact and their tendency to break in a brittle rather than ductile manner, especially when dry.
In general, improvements in the impact resistance of thermoplastic resins have been achieved by incorporating a low modulus rubber. Such a material facilitates the formation of triaxial stress distributions needed for optimum toughening. Moreover, good dispersion of the rubber phase as well as developing adhesion between the rubber and matrix contribute to efficient impact modification of these resins.
It is well known to those skilled in the art that hydrogenated block copolymers of styrene and butadiene possess many of the properties useful for impact modification of plastics. These low modulus rubber materials display a low glass transition temperature, a characteristic advantage for optimum toughening at lower temperatures. Furthermore, these block copolymers contain little unsaturation which facilitates their blending with high processing temperature plastics without degredation of the elastomer phase.
Block copolymers are unique impact modifiers compared to other rubbers in that they contain blocks which are microphase separated over the range of applications and processing conditions. These polymer segments may be tailored to become miscible with the resin to be modified. Good particle-matrix adhesion is obtained when different segments of the block copolymer reside in the matrix and in the rubber phase. This behavior is observed when hydrogenated block copolymer of styrene and butadiene are blended with resins such as polyolefins and polystyrene. Impact properties competitive with high impact polystyrene are obtained due to the compatibility of polystyrene with the polystyrene endblock of the block copolymer. Other polyolefins are toughened due to enhanced compatibility with the rubber segment.
Although the hydrogenated block copolymers do have many of the characteristics required for plastic impact modification, these materials are deficient as impact modifiers for many materials which are dissimilar in structure to styrene or hydrogenated butadiene. In particular, significant improvement in the impact resistance of polyamides with the addition of these hydrocarbon polymers has not been achieved. This result is due to poor interfacial interaction between the blend components and poor dispersion of the rubber particles. Poor interfacial adhesion affords areas of severe weakness in articles manufactured from such blends which when under impact result in facile mechanical failure.
The placement of functional groups onto the block copolymer may provide sites for interactions with such polar resins and, hence may extend the range of applicability of this elastomer. Such interactions, which include chemical reaction, hydrogen bonding and dipole interactions, are a route to achieving improved interfacial adhesion and particle dispersion, hence improved modification of polar thermoplastics.
Many attempts have been made to improve the impact properties of polyamides by adding low modulus modifiers which contain polar moieties as a result of polymerization or which have been modified to contain polar moieties by various grafting techniques. To this end, various compositions have been proposed utilizing such modifiers having carboxylic acid moieties and derivatives thereof, for example, Epstein in U.S. Pat. Nos. 4,174,358; Saito et al. in U.S. Pat. No. 4,429,076; Hergenrother et al. in U.S. Pat. No. 4,427,828; and Shiraki et al. in U.S. Pat. Nos. 4,628,072 and 4,657,971.
Epstein discloses a broad range of low modulus polyamide modifiers which have been prepared by free radical copolymerization of specific monomers with acid containing monomers. Alternatively, Epstein discloses the modification of polymers by grafting thereto specific carboxylic acid containing monomers. The grafting techniques allowed for therein are limited to thermal addition (ene reaction) and to nitrene insertion into C-H bonds or addition to C.dbd.C bonds (ethylenic unsaturation). Though Epstein does disclose a broad range of polyamide modifiers, Epstein does not disclose or suggest the utilization of hydrogenated copolymers of alkenyl arenes and conjugated dienes nor, more particularly, modified selectively hydrogenated copolymers of alkenyl arenes and conjugated dienes as polyamide modifiers.
Saito et al. disclose polyamide compositions which contain a modified unsaturated aromatic vinyl compound/conjugated diene block copolymer as a polyamide modifier. The unsaturated block copolymer has been modified by grafting a dicarboxylic acid group or derivative thereof (e.g. anhydride moieties) at a point of ethylenic unsaturation via thermal addition (ene reaction). However, such modifiers and compositions containing same are deficient in that the weatherability and resistance to thermal deterioration are poor; and, therefore, the polymers and compositions have been used only in the fields where such properties are not required. Furthermore, it is also noted that the ene reaction is a reversible reaction.
Hergenrother et al. and Shiraki et al. also describe a polyamide composition containing a block copolymer similar to that of Saito et al. However, in order to improve the weatherability and resistance to heat aging, both partially hydrogenate the block copolymer in their respective blends to an ethylenic unsaturation degree not exceeding 20 percent of the ethylenic unsaturation contained in the block copolymer prior to hydrogenation. Once the block copolymer is partially hydrogenated, the block copolymer is modified by grafting a molecular unit containing a carboxylic acid group and/or a group derived therefrom (e.g. anhydride moieties). Hergenrother et al. disclose grafting via thermal addition (ene reaction) utilizing the available residual unsaturation in the block copolymer. As such, Hergenrother et al. retained the deficiencies associated with the reversibility of the ene reaction. On the other hand, Shiraki et al. utilized free radical initiators to perform the grafting therein.
As is readily apparent in each of the foregoing prior art polyamide compositions utilizing alkenyl arene/conjugated diene block copolymers as polyamide modifiers, improved impact modification of the particular polyamide is achieved via specific interactions, between the modified diene block and the polyamide. Thus, to the extent that impact modification and strength mechanisms rely on the elastomeric properties of the diene block of the copolymer, these properties have been adversely affected by modifying the diene block in this manner.
On the otherhand, Gergen et al., in the copending U.S. patent application Ser. No. 766,215, now U.S. Pat. No. 4,783,503, describe a polyamide composition containing a block copolymer which is a thermally stable, modified, selectively hydrogenated, high 1,2 content alkenyl arene/conjugated diene block copolymer grafted with at least one functional group utilizing the metalation process. Therein, the functional groups are grafted primarily in the alkenyl arene block. In this composition, interactions between the polyamide and rubber are achieved via the alkenyl arene block.
Further research and experimentation on polyamide compositions containing the modified block copolymers of Gergen et al. in copending U.S. patent application Ser. No. 766,215, now U.S. Pat. No. 4,783,503, (K-4669) have yielded unexpected and significant impact property improvements. These new polyamide blend compositions contain block copolymers having the carboxyl functional groups present in either or both their acid and neutralized metal carboxylate salt forms. In particular, the improvement increases as the proportion of carboxyl functional groups in their acid form increases. Whether either or both of these forms in combination produce improvements may be dependent on the particular polyamide(s) selected. Furthermore, the impact properties are also improved by increasing the degree of carboxyl functionality. Additionally, depending on the particular polyamide, the particularly preferred ranges of the degree of functionality and degree of neutralization may be ascertained wherein the polyamide blend composition possesses super-tough characteristics, which are highly desirable and unexpected.
To those skilled in the art, the degree to which the grafting reaction and particle size reduction occur, thereby promoting interfacial adhesion, together with the dispersion of the rubber within the blend typically contribute to impact toughening of the blend. The results herein demonstrate that functionalizing the alkenyl arene segment promotes covalent bonding between the modified block copolymer and the polyamide. Furthermore, the block copolymer also becomes well dispersed in the polyamide phase. However, there exist examples of compositions of a modified block copolymer and polyamide which are uniquely not super-tough yet grafting and good particle dispersion are observed. Hence, the super-tough blend compositions herein are unexpected and surprising.
In the compositions disclosed herein, ionic crosslinking is present within the alkenyl arene block domains within the modifier present in the polyamide blend composition. The function of the ionic crosslinking within the modifier phase is not entirely understood as it pertains to the properties of the blend composition.
The neutralization effect herein is to be distinguished from ionic crosslinking as is disclosed in Rees, U.S. Pat. No. 3,264,272; Saito et al., U.S. Pat. No. 4,429,076; and Gergen et al., U.S. Pat. No. 4,578,429. Rees and Gergen et al. ('429) utilize ionic crosslinking solely to improve the properties of the pure hydrocarbon polymer as opposed to improving the properties of polyamide blend compositions.
Rees is limited to ionic crosslinking in homopolymer systems in which the carboxyl groups are distributed throughout the homopolymer molecule. As such, Rees does not deal with copolymers and resulting alkenyl arene domain formation. On the otherhand, though Gergen et al. ('429) addresses block copolymers, the carboxyl groups are distributed throughout the elastomeric diene block rather than the alkenyl arene blocks. As a result, the elastomeric properties of the diene block may be adversely affected; and the arene block domain phenomena is not advantageously utilized.
Saito et al. utilize ionic crosslinking to improve the properties of modified block copolymer which are to be blended with a thermoplastic polymer having a polar group thereby improving the impact resistance and hardness of the blend. In Saito et al., the block copolymer is modified by grafting maleic anhydride onto the conjugated diene portion thereof. As in Rees and Gergen et al. ('429), the elastomeric properties of the diene block may be adversely effected; and the arene block domain phenomena is again not advantageously utilized.